Tungsten is a refractory metal desirable for its large electrical conductivity, robust chemical stability, high melting point, and wear resistance. See E. Lassner and W.-D. Schubert, The Element Tungsten, Springer (1999); and C. Rollinson, The Chemistry of Chromium, Molybdenum, and Tungsten, Pergamon Press (1975). The ability to coat materials with a thin layer of tungsten would be advantageous in order to impart tungsten's chemical stability, mechanical stability, and electrical conductivity to the substrate, obviating the need for a heavy, pure tungsten object. See A. Brenner, Electrodeposition of Alloys. Principle and Practice, Academic Press (1963). Electroplating processes are inexpensive and typically used to coat substrates with metals such as zinc, nickel, and gold. See C. Mantell, Industrial Electrochemistry, 2nd ed., McGraw-Hill (1940). The electrochemical reduction of pure tungsten, however, is not achievable in aqueous solutions, due to the reduction potential residing below that of hydrogen evolution, and the low overpotential required for hydrogen evolution on tungsten oxides. See A. Brenner, Electrodeposition of Alloys. Principle and Practice, Academic Press (1963); C. Mantell, Industrial Electrochemistry, 2nd ed., McGraw-Hill, 1940; and A. Bard and L. Faulkner, Electrochemical Methods: Fundamentals and Applications, 2nd ed., Wiley, 2000. Reports of pure tungsten metal electrodeposition in nonaqueous electrolytes are limited to molten salts at temperatures over 300-500° C., requiring careful manipulation of deposition parameters and chemistry, and yielding low deposition rates. See K. Arpin et al., Chem. Mater. 23, 4783 (2011); K. Nitta et al., Electrochim. Acta 53, 20 (2007); C. J. Sequeira, J. Electrochem. Soc. 140, 2526 (1993); and A. Katagiri et al., J. Electrochem. Soc. 138, 767 (1991). Other reports demonstrate the ability to co-reduce tungsten with other transition metal elements such as nickel, iron, and cobalt in aqueous solutions. See M. Donten et al., Electrochim. Acta 45, 3389 (2000); N. Tsyntsaru et al., Surf. Coat. Tech. 203, 543 (2009); O. Younes-Metzler and E. Gileadi, Electrochim. Acta 48, 2551 (2003); O. Younes and E. Gileadi, Electrochem. Solid St. 3, 543 (2000); and N. Eliaz et al., Electrochim. Acta 50, 2893 (2005). Some of these alloys may contain as much as 70 at % tungsten metal. See N. Eliaz et al., Electrochim. Acta 50, 2893 (2005). For many applications, such as a cathode in a corrosive sodium battery, however, these alloying elements are electroactive and pure tungsten is required. See J. Kim et al., Electroanal. Chem. 759, 201 (2015); and H. Kim et al., Chem. Rev. 113, 2075 (2012).
Therefore, a need remains for an electroless process of depositing refractory metals, such as tungsten, on a substrate.